1. Field of the Invention
This invention relates to an acryl-modified polyester resin composition and a method of preparing the acryl-modified polyester resin composition. More particularly, this invention relates to an acryl-modified polyester resin composition of paints for coating a pre-coated metal (PCM) steel sheet used for home appliances and a method of preparing the acryl-modified polyester resin composition.
2. Description of the Related Art
Paints for coating a PCM steel sheet require excellent mechanical properties, chemical properties and working efficiency. The mechanical properties include processibility that represents flexibility of a film after coating, pencil hardness that indicates hardness of a coated surface, adhesiveness between the undercoat film and a substrate or topcoat film, or friction resistance. The chemical properties include an acid resistance, alkali resistance, solvent resistance or weather resistance. The working efficiency includes an operation property that determines product yields and costs according to an operating speed.
Generally, the paints for coating the PCM steel sheet include acryl based, urethane based, epoxy based, polyester based, silicon based, fluorine based and polyvinylchloride based resins or combined resins thereof. Most paints including the resins are thermosetting paints and mixtures with a curing agent.
Japanese Patent Laid-Open Publication Nos. 1996-196991 and 1997-241582 disclose paint compositions with excellent processibility. The paint compositions include a polyethylene terephthalate (PET) resin or acryl resin as a main element, and a melamine or urethane cross-linking agent for improving film hardness. However, the PET resin has poor compatibility with a solvent or a variety of additives. Thus, the PET resin does not provide uniform film appearance and also has poor processibility for home appliances.
Therefore, the polyester based, epoxy based and urethane-modified polyester based resins have been widely used in coating paints except for the paints requiring specific properties. In particular, the polyester based resin is the most commonly used for its excellent mechanical property and operation property during the coating process.
The paints for coating the PCM steel sheet using the polyester resin have been applied on a corrosion resistant alloy steel sheet such as a molten zinc electroplating steel sheet, galbanum steel sheet, aluminum steel sheet or cold zinc steel sheet. The paints have been used for exterior finishing materials for building and industry, and materials for home appliances engineering. The paint including the polyester based resin for the exterior finishing materials has weak durability to repetitive natural conditions such as ultra violet ray, acid rain, sulfurous acid gas, wetness or dryness.
The paint for coating the PCM steel sheet for home appliances requires excellent processibility and hardness. Generally, the paint for coating the PCM steel sheet for home appliances includes a polyester resin as a main element, a melamine resin or isocyanate as a cross-linking agent, and a small quantity of additives such as a curing catalyst, anti-foaming agent or leveling agent. The paint includes a linear polyester resin having a low glass transition temperature (Tg) in order to improve processibility, and to reduce cracks after the application on the steel sheet. The linear polyester resin has a lower hydroxyl value and higher molecular weight than the polyester resin for the PCM steel sheet for construction materials. Thus, the linear polyester resin has low compatibility with a solvent. The linear polyester resin gives excellent processibility for the coated film, but has poor hardness. Thus, during processing or conveying of the steel sheet, film surface is impaired from contacts of the steel sheets or manual operation by an operator.
Moreover, due to low compatibility, the paint including the linear polyester resin is changed to a suspension while storing. Additionally, layer dissociation between the solvent and the resin occurs during the wintertime. Also, a precipitation is generated. Therefore, when the linear polyester resin is formulated to paint, pigments in the paint precipitate, and a storage property of the paint is impaired. Moreover, gloss and distinctiveness of image after coating are damaged to give a coarse surface.